In conventional ultrasonic imaging, the ultrasonic probe is used extracorporeally. Therefore the operator has much flexibility in selecting the imaging plane. The operator can translate and rotate the probe as required to generate the most desirable image. However, acoustic access to some clinically interesting regions of the anatomy is limited by intervening structures. For example, in human adult echocardiology the ribs and lungs restrict acoustic access to a few standard views. Moreover, variations in body fat give rise to velocity inhomogeneities which degrade resolution in conventional ultrasonic imagers. For these reasons, esophageal echocardiography has undergone major advances in imaging the crucial central area of the heart. The left atrium, pulmonary veins, mitral valve area, subaortic area, and pulmonary arteries are all in close proximity from a posterior approach to probes placed in the esophagus. The internal probe thus avoids some of the hindrances to acoustic access; however, it also compromises the operator's ability to position the scan plane. Especially in congenital heart disease, where the anatomy is likely to be nonstandard, this limitation on scan plane selection can be a significant clinical problem.
A probe capable of generating several image planes from a known source position would be extremely useful in esophageal echocardiography. The ability to generate several image planes from a known source position would make it possible to reconstruct the data in three dimensions. Advances in three-dimensional medical imaging allow a series of two-dimensional images to be converted into three-dimensional renderings. One especially useful kind of presentation is the reconstruction of three-dimensional surfaces from the two-dimensional data. One approach to this problem as set forth in commonly-assigned U.S. Pat. No. 4,719,585, issued Jan. 12, 1988, is to use a "dividing cubes" algorithm which can be easily implemented by a digital signal processing accessory board for a computer workstation. Alternatively, software algorithms are well-developed for viewing objects from arbitrary directions and for selectively removing arbitrary volumes or surfaces to enable visualization of internal structures.
Esophageal echocardiography has been performed with a single linear phased array mounted near the probe tip generating a single, usually transverse, scan plane. The scanning limitations imposed by this arrangement have resulted in efforts to produce a biplane probe. These efforts led to fabrication of biplane esophageal probes with two right-angle oriented arrays at the tip (one transverse and one longitudinal), each with a separate wiring harness and consequently a large cable bundle that limits the probe's maneuverability. While producing two independent images at different locations, the increased probe size makes the scan plane selection even more difficult. There is an ongoing and continued need for continuous alteration of the scan plane of imaging, especially for clinical assessment of some forms of congenital heart disease.
Mechanical systems have been suggested involving angulation, or pivots of the tip of a phased array probe, for advancement or retraction of the probe as it moves along the esophagus, to localize the scan plane. These systems have not been clinically successful because the elasticity of the esophageal tissue tends to oppose the desired motion, leading to locally distorted anatomy and less displacement than expected. Such problems can be expected when the probe tip must move either axially or laterally along the surface of the tissue.